Add aggregation, binning, granularity, sort, and stacking to the Chart Builder

- Per-channel transforms: aggregate (sum/mean/median/min/max), quantitative
  bin, and temporal timeUnit granularity; bin and aggregate are mutually
  exclusive. A field-less "Count of records" measure (Voyager's count(*)).
- Chart-level sort (rank a categorical axis by its measure) and stacking
  (zero / 100% normalize), each shown only when it applies.
- Field type is a fixed N|O|Q|T segmented control with the column's invalid
  types disabled; SegmentedControl gains APG-correct disabled options.
- A crowded-category-axis warning (a raw measure drawing one mark per row over
  a large dataset) and a disabled-Create hint (says why it's disabled).
- Drop the Create success toast — the new snippet is immediately visible.
- Docs: spec §06, research-doc §8 backlog (incl. the cardinality/extent
  profiling TODO), architecture 01 (stable-selector rule) and 05 (builder-local
  preview), and a profiling breadcrumb.

docs/IMPLEMENTATION-PLAN.md

@@ -284,6 +284,11 @@ the reference.
 
 **Goal:** no-JSON chart composition from a dataset → a new snippet.
 
+> **Enhancement backlog** beyond the Tier-B floor (aggregation, binning, stacking,
+> temporal granularity, sort/orientation, cardinality-based warnings, Tier C
+> intent-first) lives in [`docs/chart-builder-research.md`](chart-builder-research.md) §8
+> — its single home, so these stop living in chat.
+
 **Core**
 
 - `chart-builder.ts` — pure spec assembler: (mark ∈ Bar/Line/Point/Area/Circle) +